1. Field of the Invention
The present invention relates to a photoresist which is used for photolithography to form patterns inclusive of fine patterns equal to or smaller than in size the light wavelengths of the exposure light, and the photolithography using thereof.
2. Related Background Art
Recently, it has become indispensable to make photolithography further finer, with the developments of large capacity semiconductor memories and speed-enhancement and large-scale integration of CPU processors.
The lights used in photolithography apparatuses have continued to become shorter in wavelengths as means for making photolithography finer, and at present near ultraviolet laser lights are used and hence it is possible to make microfabrication of the order of 0.1 μm.
In order to make photolithography much finer, however, there are many problems to be solved, including the further shortening of laser wavelengths, development of lenses usable in such wavelength regions, miniaturization of equipments, etc.
On the other hand, methods which use near-field light have been proposed for the purpose of optical manufacturing of photoresist patterns with widths not larger than the wavelengths of the lights used.
For example, Japanese Patent Application Laid-Open No. 7-106229 discloses a method for near-field exposure based on a probe scanning which uses a probe made by sharpening a tip of an optical fiber by wet etching.
In addition, for the purpose of solving the problem that the above-mentioned method is slow in throughput, many proposals such as Japanese Patent Application Laid-Open No. 11-145051 are made on the en bloc near-field exposure with photomasks.
The merits provided by near-field exposure are that the minimum fabricable pattern width is independent of the wavelength of a light used, but is determined by the aperture of the probe and photomask used. Thus, if a semiconductor laser, for example, is used as a light source for exposure, there is a merit that the apparatus can be made smaller owing to the extremely reduced size of the light source, which also reduces the unit cost of an exposure apparatus.
Accordingly, since the exposure sensitivity of the photoresist is effective in the exposure light wavelengths of about 200 to 500 nm, a blue semiconductor laser can be used as an exposure light source within this range to make the apparatus compact. Alternatively, a general-purpose mercury-arc lamp can be used to provide the exposure light of a high output power, where it is preferable to use the g-line and i-line photoresists in consideration of photoresist sensitivity.
Furthermore, the g-line and i-line photoresists are at present used as general-purpose materials, large in variety, easily available, and inexpensive, and hence there is a merit that the degree of freedom for process is high and the cost can be reduced. Since in the near-field exposure, the width magnitude of the fabricable pattern is not limited by the light wavelength used, there is a possibility that the microfabrication can be made with the g-line and i-line photoresists.
As the g-line and i-line photoresists, the alkali-soluble novolak resins containing a compound comprising the naphthoquinone diazide group as a photosensitive agent have long been used. In Japanese Patent Application Laid-Open No. 7-319157, an example for the above-mentioned photoresist is disclosed, where a high etching-selectivity is shown to be generated in the patterns obtained by the i-line exposure. Since commercially available g-line or i-line photoresists, inclusive of the above-mentioned case, are supposed to be used in the conventional photolithography that employs steppers and aligners, that is, in a method that employs for exposure a light passing through apertures of a photomask but not in the near-field exposure, the minimum pattern width corresponds to the resolution of the order of several hundreds nm to several μm, and the photoresist-film thickness is usually set to be about 0.5 to 1 μm or larger. For example, in Japanese Patent Application Laid-Open No. 7-319157, a case of the film thickness of 1.5 μm is disclosed.
In the conventional photolithography methods, mercury-arc lamps and excimer lasers are used as exposure light sources, so that the exposure intensities fall in the range of several tens to several hundreds mJ/cm2. The photoresists used are required to have sufficient sensitivities for these exposure intensities, and to have such film thicknesses that they can tolerate etching in substrate processing subsequent to pattern formation.
In the near-field exposure method, however, the photoresist-film thickness cannot be as thick as those in the conventional methods. The reasons for this will be described below.
In the near-field exposure, since a photoresist is exposed to the scattered light produced by disturbing the near-field light with the photoresist, there is observed a tendency that the large thickness of the image-forming photoresist layer results in the large widths of the formed patterns. This is illustrated in FIGS. 4A and 4B, where reference numeral 204 denotes a mask base and 205 denotes a light shielding film.
By making the exposure light 505 stream into the photomask having microapertures 513, the near-field light 510 is formed in the neighborhood of a microaperture 513 (FIG. 4A). When the photomask and the photoresist 503 are brought closer together (FIG. 4B), the near-field light 510 is scattered by the photoresist 503 placed on the substrate 504, the reacted photoresist portion 501 is then formed in the photoresist 503. When the photoresist film is thick, the extension of the reacted photoresist portion toward the substrate 504 is enhanced, resulting in the broadening of the fabricable pattern widths. When the intervals of the microapertures are small, the reacted photoresist portions resulting from these apertures overlap each other, providing a much broader line width of the formed pattern. Accordingly, an embodiment with large photoresist-film thickness can not make the best use of the merit of the near-field exposure. In order to take advantage of the merit of the near-field exposure, the film thickness of the photoresist is desirably smaller than the mask aperture diameter which provides near-field light.
Since the lithography using near-field light aims at such micropattern formation that cannot be obtained by the conventional methods, in general the smallest dimension of the mask aperture is not more than 100 nm. Accordingly, the film thickness of the photoresist should be not more than 100 nm.
With such a small film thickness, however, pattern shapes after the exposure and development of the photoresist tend to be nonuniform. In other words, the edges of the patterns do not follow the prescribed lines or curves but have irregularities. The irregularities, that is, the pattern edge roughness is due to the photoresist remaining as aggregates of the order of 10 μm in diameter after development. They adversely affect the dimensional accuracy in the patterns finer than 100 nm to cause problems.
The present inventors used the above-mentioned commercial g-line positive photoresist to conduct the near-field exposure to make the patterns of 200 nm in pitch and 70 nm in line width with the exposure light at 442 nm wavelength, and observed the sectional shapes by a SEM to find that the pattern edge roughness was large and in addition the rectangularity was poor, which rectangularity will be explained below.
In general, it can be said that the magnitude of the pattern edge roughness acceptable in device fabrication is 10% of the pattern width, while there occurs fierce pattern edge roughness in the patterns made as mentioned above, possibly giving rise to adverse consequences in device fabrication.
By the way, the pattern edge roughness concerned is defined in terms of dispersion of the widths of the fabricated patterns as follows.(Pattern edge roughness)={(maximum width for the fabricated patterns)−(minimum width for the fabricated patterns)}/(assumed pattern width)
For instance, when the widths of the patterns fabricated with the assumed pattern width of 1 μm spread from 0.9 μm to 1.1 μm in difference, the pattern edge roughness amounts to(1.1−0.9)/1=0.2
From the above equation, the pertinent pattern edge roughness is found to be 20%. The pattern edge roughness was 50% for the above-mentioned fabrication of the patterns having a line width of 70 nm.
In addition, in the present proposal, as a method for numerically representing the precision of the fabricated patterns, the “rectangularity” quantity is defined as follows:(Rectangularity)={(assumed pattern width)−(magnitude of the “shear droop” in a fabricated pattern)}/(assumed pattern width)
For instance, for such patterns as shown in FIG. 6, the assumed pattern width 602 is 100 nm, the magnitude 601 of the shear droop of the fabricated patterns is 20 nm, resulting in a rectangularity of 0.8, that is, 80%. In FIG. 6, reference numeral 103 denotes a photoresist and 104 denotes a substrate.
The rectangularity was found to be 50% for the above-mentioned fabrication of the patterns having the line width of 70 nm.
In the present proposal, the object is to fabricate patterns with the rectangularities not lower than 80%. With the rectangularity lower than 80%, the subsequent process tolerance is diminished, resulting in the throughput lowering and cost rising with a high degree of likelihood.
Efficiency for light utilization in the near-field exposure will be explained below.
The absorption coefficient α (μm−1) of the photoresist measured by the present inventors with a laser of the 442 nm wavelength was 0.08, which photoresist is a g-line positive photoresist commercially available and assumed to be used in the exposure processing of a film of about 1 μm in thickness and the minimum pattern width of about 450 nm, by use of a stepper and an aligner.
Accordingly, when the photoresist is applied to the substrate so as to make a film of 1 μm in thickness, the transmittance of the resulting film is 92%. When the same photoresist is applied to the substrate so as to make a film of 100 nm in thickness, the transmittance of the resulting film is 99%.
A large amount of transmitted light means that most part of the light transmits through the photoresist layer without being absorbed, resulting in boosting the possibilities that the rectangularity is deteriorated due to the perturbation of the pattern side wall shape by the reflected light from the photoresist-applied substrate, the pattern edge roughness is enhanced, and the like.
Thus, the use of the commercial g-line and i-line photoresists, as they are, which match to the pattern formation methods such as the reduction-projection exposure method in which imaging is made by means of lenses and patterns are formed on photoresist films having a film thickness of the order of 1 μm, etc., results in a low “efficiency for light utilization” in the near-field exposure. As mentioned above, since the light passes through the microaperture of a probe resulting in a significantly reduced light transmittance, and furthermore only about 1% of the transmitted light contributes to the exposure, a ratio of the light contributing to the exposure to the incident light is very small. Furthermore, there is a fault that there occurs such fierce pattern edge roughness that gives adverse results in device fabrication.
In particular, in the present specification, the degree of contribution to exposure of the near-field light generated by the microaperture, that is, the degree in which the exposure making the photosensitive compound in the photoresist cause the photochemical reaction is referred to as the degree of the “efficiency for light utilization” of the exposure light.